1. Field of the Invention
This invention relates to an improved procedure for evaluating the performance of at least one apparatus in a pool of like apparatuses. Such performance evaluation lies within the category of quality assurance or QA and quality control or qc. In the field of biomedical and laboratory equipment and apparatus, the QA can be accomplished by comparing the performance of a specific type of equipment or apparatus, located in one laboratory, with the performance of like apparatuses in other laboratories, often in other cities and states. Such an evaluation program is called an Interlaboratory Quality Assurance Program-IQAP.
The field of this invention is directed to statistical comparisons of apparatus performance. Accordingly, to set forth the invention in a meaningful environment and best mode, a specific type of laboratory apparatus needs to be identified. One such apparatus is a blood analyzer sold by Coulter Electronics, Inc. under the trademark COULTER COUNTER.RTM. and is exemplified by U.S. Pat. No. 3,549,994. Such an apparatus was called the Model S and has been modified and improved over the past seventeen years to become two full series of semi-automatic hematology analyzers, well known throughout the world.
The need for a highly informative and readily usable IQAP for laboratory equipment, especially hematology analyzers, is self-evident. These apparatuses are complex and provide quantitative data on numerous blood parameters, for example: the red blood cell counter (RBC), hematocrit (Hct), hemoglobin (Hgb), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), the mean corpuscular hemoglobin concentration (MCHC), and the white blood cell count (WBC). Definitions of each of these parameters are set forth in U.S. Pat. No. 3,549,994. Quantitative data also can be obtained from other components of the blood, for example, platelets, neutrophils, eosinophils, lymphocytes, and monocytes, by use of current generation COULTER COUNTER analyzers.
Quality control for hematology equipment is a relatively complex issue for two essential reasons, the first being the lack of standards, i.e. materials with a known number of analytical variables; and the second being the number of analytical variables that can affect test result performance. Other significant factors affecting the relative accuracy of test results can be (1) changes in calibration, (2) specimen stability or shelf life, (3) reagent condition, and (4) instrument performance.
Many hematology apparatus quality control techniques have been utilized in the laboratory and are in use today. They are: calibration, commercial controls, XB analysis (called "X bar B"), interlaboratory comparison, and technologist review.
Calibration is a method for achieving or "setting" instrument accuracy, namely by adjusting the instrument to duplicate a single assay value of a calibrator, such as fresh whole blood, values of which were assigned by a reference method.
Commercial control, while satisfactory, presents a problem of formulation which can make a substantial difference in control stability and performance. The impact of this problem, to a large degree, can be lessened by matching the controls to the instruments and utilizing compatible reagents.
XB analysis is a statistical technique utilizing patient results for continually monitoring COULTER COUNTER analyzers and other apparatus. As conceived by Dr. Brian Bull of Loma Linda University, the XB algorithm is based on the demonstrated stability of the erythrocyte (red cell) indices, mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC) of select patient populations.
Daily instruments checks monitor calibration stability, reproducibility, background counts for confirming the proper functioning of reagent level, diluting and dispensing systems, and general instrument performance.
Interlaboratory comparison programs objectively establish the quality of a laboratory's performance referenced to peer performance for maintaining high performance levels in accordance with the various accreditation criteria of many state, regulatory, and professional agencies.
While each of the above techniques are satisfactory and have permitted achieving a hematologically acceptable degree of control over the accuracy of analytical apparatus, thereby obtaining accurate information about a blood sample, each does suffer from some inherent disadvantages as follows: Whole blood calibration is time consuming. It requires a large quantity of fresh whole blood. As pointed out by Gilmer and Williams in "The Status of Methods of Calibration in Hematology", Am. J. Clin. Path. 74(4): 600, "there is always the real possibility that the mean value obtained may not be accurate". Commercial controls can lead to serious analytical errors through improper use, in storage and handling. Furthermore, there are many circumstances where controls are not practical. XB analysis is not applicable when patient populations are non-representative so as to generate outlying indices, i.e. neonates and oncology patients. Furthermore, XB only monitors ability of original calibration, which could be in error. Also, other generally measured parameters, such as with white cells or platelets, are not monitored by the XB analysis. Professional interlaboratory comparison programs often lead to erroneous interpretation because of specimen/instrument incompatability. The use of a technologist review is at best arbitrary and depends on the skill level of the technologist.
2. Description of the Prior Art
Statistical quality control systems, which aim at determining the statistical distribution of certain quality characteristics of a sample, are disclosed in U.S. Pat. No. 3,151,237. Other types of statistical systems are disclosed in U.S. Pat. No. 4,320,463, in "Medical Research" by John M. England in Chapter 1 thereof, and in "Statistics in Medicine" by Theodore Colton, Sc.D. published by Little, Brown and Company, Boston, Eight Printing (QA-276-Co-1974).